A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. In an embodiment, the liquid is distilled water, although another liquid can be used. An embodiment of the present invention will be described with reference to liquid. However, another fluid may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
In lithography, the manner of illumination of the patterning device, in particular the angles at which the exposure radiation is incident on the patterning device, affects the image printed on the substrate. Illumination modes are commonly, and most conveniently, described by describing the distribution of radiation in a pupil plane in the illumination system that is a Fourier transform of the patterning device plane. Thus, position in the pupil plane corresponds to angle at the patterning device plane. So-called conventional illumination has a disc of radiation in the pupil plane centered on the origin and results in the patterning device being illuminated with radiation arriving at a range of angles centered around the normal. Annular illumination has an annulus of radiation in the pupil plane. Off-axis multi-pole arrangements have, commonly, two or four poles arranged symmetrically about the origin, either on or off the X and Y axes. These different modes are known to be suitable for different types of pattern to be imaged. Complex patterns may require a complex illumination mode for optimum imaging.
A diffractive optical element (DOE) can be used that is custom made and placed in the illumination system in a plane conjugate with the patterning device but closer to the radiation source than the pupil plane in order to define a desired illumination distribution in the pupil plane.